Presented below is background information on certain aspects of the present invention as they may relate to technical features referred to in the detailed description, but not necessarily described in detail. The discussion below should not be construed as an admission as to the relevance of the information to the claimed invention or the prior art effect of the material described.
Exhaust gases from many combustion processes are contaminated with carbon dioxide when emitted into the atmosphere, which contributes to global warming and environmental damage. Such gas streams are difficult to treat in ways that are both technically and economically practical.
Gas separation by means of membranes is a well-established technology. In an industrial setting, a total pressure difference is usually applied between the feed and permeate sides, typically by compressing the feed stream or maintaining the permeate side of the membrane under partial vacuum.
Although permeation by creating a feed to permeate pressure difference is the most common process, it is known in the literature that a driving force for transmembrane permeation may be supplied by passing a sweep gas across the permeate side of the membranes, thereby lowering the partial pressure of a desired permeant on that side to a level below its partial pressure on the feed side. In this case, the total pressure on both sides of the membrane may be the same, the total pressure on the permeate side may be higher than on the feed side, or there may be additional driving force provided by keeping the total feed pressure higher than the total permeate pressure.
One such sweep-based membrane separation process for treating an exhaust gas is presented in co-owned U.S. Pat. No. 7,964,020. A simple flow scheme for that process is shown in FIG. 1.
Referring to this figure, fuel stream 101, and air, oxygen-enriched air or oxygen stream 113, are introduced into a combustion step or zone, 103. Stream 113 is made up of sweep stream, 112, discussed below, and additional air or oxygen supply, stream 102.
Combustion exhaust stream, 104, typically containing 10-20 vol % carbon dioxide, is withdrawn. The stream is sent, at least in part, to carbon dioxide capture step, 105, that produces a concentrated (greater than 60 vol %) carbon dioxide product stream, 106, from the exhaust stream. The concentrated stream, 106, may be further concentrated in a second step and sent for sequestration, or used or disposed of in any other appropriate way.
The off-gas stream, 107, from the capture step still contains carbon dioxide, at a lower concentration than the raw exhaust stream. This concentration may be up to about 10 vol % carbon dioxide for coal-fired boilers, lower for gas-fired boilers. The off-gas stream is sent for treatment in membrane separation step or unit, 108. The unit contains membranes, 109, that are selectively permeable to carbon dioxide over nitrogen and to carbon dioxide over oxygen.
The off-gas flows across the feed side of the membranes; a sweep gas of air, oxygen-enriched air or oxygen, stream 111, flows across the permeate side. The sweep stream picks up the preferentially permeating carbon dioxide, and the resulting permeate stream, 112, is withdrawn from the membrane unit and is combined with stream 102 to form the air or oxygen feed, 113, to the combustor.
The residue stream, 110, is reduced in carbon dioxide content to less than about 5 vol % and is typically discharged to the environment.
Some of the additional beneficial consequences of using the combustion air or oxygen supply as the permeate sweep in this process is that the permeating carbon dioxide removed into the sweep gas is recycled to the combustion chamber. This increases the carbon dioxide concentration in the exhaust gas leaving the combustor, facilitating the downstream capture of carbon dioxide.
Despite the advantages of the above process, at some locations, there are several exhaust streams that contain carbon dioxide, which are produced from multiple combustion processes or units, such as boilers, furnaces, or ovens. As such, implementing multiple carbon capture systems to treat the exhaust streams from each combustion source would be expensive and capital intensive.
Additionally, the above process requires that the operating conditions of the combustion unit be modified since the air stream entering the combustion unit is diluted, resulting in a reduced concentration of oxygen compared to its conventional operation. Since most combustion units are optimized to work within a specific range of oxygen content, any decrease in oxygen levels would affect the efficiency of the combustion unit and the overall plant. For example, when this process is applied to coal power plants, studies have shown that the power plant boiler can operate with an air combustion stream containing as little as 18% oxygen. However, even after the boiler has been modified to work with this oxygen content, the plant will suffer a 1-2% reduction in efficiency. Similarly, natural gas power turbines can operate with as little as 15% oxygen, but again, a reduction in power output results.
Thus, it would be beneficial to develop a single process that is able to treat multiple exhaust gas streams from different combustion sources where a sweep-stream is recycled back to only one of the combustion sources. In this way, the cost of modifying the combustion unit and the loss in efficiency mentioned above would only be incurred by one combustion unit although the carbon dioxide would be captured from several.